HP 5061A Atomic Clock Repair

HP 5061A Atomic Clock Repair

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[Marc] Argh! And just when I was  ready to do a two clock experiment,   the second clock, I can't start it, and  it shows the yellow light of distress! Hello and welcome back. Yep, you heard right,  my precious atomic clock just failed. Actually,   it’s not really mine, it’s on loan from team  member and uber collector Marcel. Sorry Marcel,   I broke your clock. However,  looking at the bright side,  

I’m going to try to repair it in this episode,  which is probably going to be entertaining. So, if you follow us, you know  that we have already played with   Marcel’s HP 5061 atomic clocks in the  past. We explained how they worked,   and fired one up in a 2020 video. Then, in  2022, we brushed up on our quantum mechanics,   and fine tuned them using the  Nobel-price worthy Zeeman effect.

At which point we ended up with not just  one, but two tuned atomic clocks on the   bench - and Marcel had two more,  although not working at this point. Hmm, did I just say that we  have several atomic clocks? What about using them to check Einstein’s  theory of relativity for ourselves? Einstein’s relativity makes two predictions  that should affect our ultra-precise clocks:   that times changes if clocks move at a different  speeds relative to each other, or if they   experience different amounts of gravity. That  said, unless you move at a reasonable fraction   of the speed of light, or go to a super heavy  planet, these changes are infinitely small. However, in 1971, scientists Hafele and  Keating realized that this effect might   finally be measurable, thanks to the new  HP 5061A clocks, the very ones we have.  

In a world’s first experiment, they flew  them around the earth in opposite directions,   and proved that the clocks had indeed  sped up and slowed down in accordance   with Einstein’s theory, the first  direct proof of time relativity. More recently, amateur Tom Van  Baak, a self-described time nut,   managed to re-run the experiment with the improved  HP 5071A clocks, the modern successors to ours. He took his 3 clocks, and his 3 kids, up Mount  Rainier near Seattle, for a couple of days,   then compared them to his Hydrogen Maser  master clock, that he also happens to have   at his home. Did I say he was a proper time nut?  Take a look at his HP clock collection. Anyhow,   he was able to measure a 20 ns jump in  time due to the lesser gravity at higher   altitude. Although it was far less than  the 273 ns that was obtained in the Hafele   and Keating experiment, it was still measurable. It’s going to be even harder to do  with our old and tired HP 5061A’s,   which by the way are all from eBay, but  we thought it’d be fun to give it a shot.

To see if it’s even possible, I need to see how  closely the two clocks can track each other,   when they are sitting next to each other  in the lab. I’d need them to stay stable   to within a few ns over several days to have  any hope of measuring a relativistic shift. So, that’s what I was setting up to do,  when, predictably, one of the clocks failed.

So, no measurement today. Instead,  we need to dive into the innards   of this very intimidating quantum  monster, and see if we can save it. [Marc] I can't start it, and it  shows the yellow light of distress. And fortunately, I don't think it's  with the tube. Because the tube is   behaving properly, I think. Where is the ion pump?

My ion pump current decreased all the  way to zero. But I have no multiplier,   and no beam signal. So, if there's no beam  signal, I can't measure anything of course. I think that's the fault: that's  the multiplier. On the other clock,   it is at a healthy... whatever...  multiplier... healthy 30. At this point, we need a short bout of  elevator music to explain what I think   has gone wrong. It’s explained  in full detail my 2020 video,   link in the doodly-doo, but I’ll just  summarize what’s important for this video.

All of the atomic magic happens  within a sealed vacuum tube,   which I hope is still functioning correctly. Inside this tube, an oven heats up Cesium atoms,   which evaporate and form a beam that goes through  magnets and a microwave cavity. The first set of   magnets selects only the atoms which outer  electron is in the +1/2 spin quantum state. Meanwhile, a strong microwave signal is fed  into the following cavity. Due to quantum   mechanics magic, when the signal is exactly at  9.192,631,770 GHz, and only then, the spin of   the Cesium atom is changed to the opposite -1/2  quantum state. Another magnet at the output of  

the cavity then selects all of the -1/2 quantum  state Cesium atoms, and sends them to a detector. The result is that we get an incredibly  narrow peak signal out of the tube,   when we hit the magic 9.192,631,770 frequency on   the nose. This frequency also happens  to be how the atomic second is defined. An electronic feedback loop continuously tunes  the frequency of the microwave oscillator,   to maintain the signal out  of the tube at its peak,   guaranteeing that the microwave signal is  always at the perfect frequency. It is then   divided to give the 5 MHz reference  frequency output of the instrument. You can therefore easily imagine, that a  large part of the instrument is devoted to   the generation of the microwave signal, at 9.192  etc... GHz. This is done by starting with a 5 MHz  

crystal, and using a combo of frequency synthesis  and frequency multiplication, as shown here. It all starts with the 5 MHz oven-stabilized   quartz oscillator. The 5 MHz is then fed  to a x18 multiplier, generating 90 MHz. The 90 MHz is then sent to a harmonic  generator, a piece of RF magic this time,   which generates harmonic multiples of the 90  MHz, extending far into the microwave spectrum. The 102nd harmonic is then selected by  a resonant cavity, and sent towards the   tube. But that only gets us to 9.180 GHz. Close,  but not quite. We are still 12.631,770 MHz off. And that’s why you see a digital synthesizer in  the diagram, which generates the 12.631,770 MHz  

we are missing. This is also fed into the harmonic  generator, and, because of more RF magic, it gets   added to the 9.180 GHz frequency, resulting in  the 9.192,631,770 GHz we needed at the output. All that so say that I think there is a fault in  this circuit. Conveniently, HP added a detector at   the input of the A4 harmonic generator, that  indicates the level of the 90MHz coming out   of the A3 multiplier on the front panel, when it  is in the aptly named “multiplier” position. And   that level reads almost zero. That’s no good. And  that’s why I suspect that the fault is with either  

the A3 multiplier, or on the input side of the A4  harmonic generator. Which, while not particularly   amusing, is a lot better than a fault in the  irreparable, and immensely expensive Cesium tube. [Marc] So, I think a fault has  happened in the multiplier. Oh joy,   oh joy. So, we need to repair that one.

So, fortunately, this instrument  is superbly well documented,   in the 300-plus page manual, which I have  scanned, so everybody has access to it. And, if I look at it, it gives me  a hint of where the fault might be. So, there's a whole chain here  that starts at 5 MHz to make,   eventually, the 9.1 92 GHz to  match the line of the cesium. So, it starts at 5, then there is a multiplier  up to 90 MHz, and there's a harmonic generator.

Simple enough if you look at the block  diagram, but more complicated in reality. So, here's our oscillator - I know it works,   because I get 5 MHz - and there's the 90  MHz generator, which is a simple multiplier. But the error comes from module A4, which is  the harmonic generator that does the times   102 multiplication, plus it adds another signal  from a synthesizer, to do the remaining digits. So, the error comes on that yellow signal,  that's the measurement point. And that's A4.

So, our problem could be in this beautiful board,  which is the multiplier, with its schematics here. And it's fairly straightforward.  It's a times two to do 10 MHz,   then time three to do 30 MHz, another time  three to do 90 MHz, and a power amplifier. Or, it could be from the next module up,   which is the RF multiplier, which has  a step recovery diode hidden in here. And that's where our error  signal comes, from here. We are not getting enough signal. And that's an  indication of several things. It's an indication  

of both how much 90 MHz is coming in, but also  of the bias condition of the step recovery diode. So, if the step recovery diode is  open or short, this won't be correct. So, I hope, it's the 90 MHz that's not  right. Because, if not, it says this is   not repairable. This is a factory thing, we'd  have to steal it from another instrument. all right so here's the beauty and I think yes not it is three assembly multiplier  so is this guy okay RF connection on that side and power connections on that side so  this is the 5 MHz to 90 mahz and then it   gets connected up here to the uh to the step  recovery diode harmonic generator which gets   into the tube and and there are two input  to this thing there is the 90 MHz uh which   gets multiplied by 102 and there's the 12  whatever megahertz which Sy synthesized   oh G1 is this cable here oh indeed it comes  from the side of the multiplier so it's this   fellow that we somehow have to extract then  connect to something else all right I got it okay so first suspect yeah and that's  the output of e3 so I need to find a way   to connect myself to it okay there was  a little challenge because this is an   SMB connector which is not that usual  so I had to make a little adapter and   hopefully I can get that in and we  are going to hook it up to my param meter and we should get 25 dbm out of that  thing so let's plug it in I here the high   voltage Supply and then let's turn that on  we have nothing we have we went from what 23.7 to 22 it's our multiplier that's not  putting enough power out uh let's check   that on the scope now that we know  that we don't have much power coming   out of it safe to get it attached to the  scope okay so let's see what that [Music] does all right well we have  something trigger trigger trigger   trigger 4 M molt Peak to Peak so that's why  we have such a low signal and then we want frequency yeah it's 90 MHz so that's  the correct thing but it's only 125 MTS   and it should be 4vt RMS so there's  something wrong with my multiplier so sort of good news bad news bad news we  don't have enough 190 MHz but good news   it's in the assembly that's easier  to debug it's not the RF uh assembly [Music] oh wait oh oh oh oh I see here is our module power on that side and then  so 5 MHz in 5 MHz out and 132 Hertz   phase modulation which we don't need that's for locking oh I see some metal gaskets  in there oh yeah there we go that's our metal gaskets and this look pretty straightforward no similar complexity to our  Apollo stuff very linear a lots of tripler   and doublers so lots of coils to tune it might be  just out of adjustment although that far seems a   little bit much soon after so I have hooked up my  multiplier module over there I feed it with 5 MHz   from this guy which we repairing can't remember  in which episode and this has a lot of oomph so   I I can get the 3vt peak to Peak which my uh scope  generator cannot do and then I give it 17 no 18.7  

volts from over here on the terminal and I look  at the output and uh if history repeats itself   we should see that we have almost no output 1 2  3 and we have plenty of output we have so much I   can't measure it so I need to put an attenuator  but something is odd it works so I suppose the   reason I don't have any output is because I don't  have enough input I need to go back in the machine   and look at uh what we get out as an input a  5 MHz input uh why I'm at it I I'll retune it   it's it's not that difficult and I suppose  it's pretty well tuned a few minutes later   so it's all tuned uh I put an anator so I could  measure it so now I am at 1.47 so I'll have about   uh 3 volt peek to peek about twice that because  of the attenuator and it was all pretty much in   tune except this coil they give you complicated  procedure but in a large multiplayer like this   it's just easier to tweak the cords until you  maximize the signal pretty simple so I gain   maybe 30% something like that so this has a clean  bill of health uh the fold is somewhere else so   we're back in our instrument I hooked myself up  to what was going to J2 which should be the 5 mahz and would don't you know it we have tons  of signal we have exactly what we should have   1V RMS at 5 mahz and which is about 3  volts Peak to Peak so why didn't we get   our output out of the multiplier that is odd  maybe pluged in this some plugs weren't working   or something let's plug it back in all right  so I've reconnected my multiplier and should be   on the scope and we have our 90 mahz back but we  haven't done anything he just came all by itself maybe there was a connector problem  let let's measure how much output we have so this I know what to expect I put a 6B   at meter so you got you guys watch  this you're going to see before me [Music] and it's 60 dbm plus 6 so I'm at 22  I should be at 25 I am 3db short which I see   on the scope too I should be at 4 Vol RMS but  I am 2v RMS so it's a little bit short um and   I also see that in the uh there is a resistor  to select the gain and they went to the the to   the minimum of that resistor which maximizes  the gain so they have trouble getting to the   voltage to before so I don't know if I need to  worry about it change a few transistors in the   multiplier or just be happy I reced my multiplier  to the harmonic generator so we're are now in a   functioning condition and I have this on mold so  if we turn it on M should have come back it has not but I have 19 mahz so we are War fears have uh become true  with something in the harmonic generator it   does receive the 1990 mahz there's something  wrong with the the SRD module and the problem   is that the harmonic generator is wedged  in there it's this thing right here it's   another piece of RF magic right you come in at  190 MHz you come out at 9 1 GHz I would think   maybe the transistor's gone wrong the regulator  that would be our hope that's the part they   basically tell you it there's nothing for you  in there uh send it back to factory uh none of   these adjustments are adjustable except this  one which is not our problem so we are a bit   little bit in trouble okay I think I found a way  to do this you can actually disconnect the thing   and rotate to tube and how it comes so this has  been disconnected Ed and I left the high voltage   but there's enough that I I can go in there and  hopefully check the transistor that's probably   the only thing I can do in that assembly uh and  if that if this not the transistor then we have   to go back and work on the multipliers right  now it's putting half out of what it should U   put so that that might be the the root of the  problem is nothing is wrong in the multiplier all right same thing is nice gas getting around it so you can see this thing is a uh piece of  RF magic the DI is probably in here somewhere   the SRD diode step recovery diode then  um this is the resonant cavity to pick   the one2 harmonic and then there's all kind of uh  impis adaptation here there is an attenuator here   and two stubs at least three tune stubs  in it so uh would be quite difficult to retune so transistor checking um 6 good  transistor 6 transistor seems okay nothing   the direction okay so that looks like a good  transistor I also wiggle the pot in case it's   become oxidized that's for the adjustment of  bias and then the cap is super hard to get   to so I also checked the cap and it's the better  than any caps I have I think that's a talum it's   it it has all this capacitance and its series  resistance is in the hundreds of Milli ohms so   it's an excellent cap so I put it all back  together and I think it helped a little bit now I have an indication of 12 I had like3  before so whatever wiggling the pot or um moving   Connections In and Out helped um if I get it above  20 I am back in the regular operation mode so I'm   thinking that if I get the multiplier to double  its output I'll get there so let's work on that   uh because the multiplier output is definitely  half of what it should be so the way this thing   works is supposed to be automat automatic gain  control and four 4 Vols RMS here and it comes   back here is detected and here's the regulator  and however there is a factory selected resistor   uh that's used to make sure this is at 4 volts  and when I measure mine it's all the way to the   maximum is this guy over here this guy over there  and so the all the way to the maximum gain which   is actually the minimum value of this resistor so  this is always at Ground I have always the maximum   amount of power which is not enough so which tells  me they already had problem at manufacturing uh   this assembly wasn't producing enough power I am  thinking it must be one of the transistors that's   a little weak and they are 2 and 708 they're  really old weaklings uh transistors I have some of   the power transistor to replace them those are 800  MHz transistors here we we can try to replace them   see what happens 1 hour later so I've completed my  change of transistors I didn't change them all uh   I changed this one this one this one these two  are original it work worse when I change those   two they are must be very sensitive to impedance  uh and it gain me a modest amount of power and   the one that gave me the most are the two power  transistors which actually I uh figured out had   been changed before so this one and you cannot see  yeah this one over here these were not original   Originals were the right type but they were not  each they had not HP markings and I have recovered   uh 50% more power I don't have exactly two volts  I have 1.73 volts remember this is attenuated by   factor of two so it's um it's pretty close I  I don't I have like 3.5 volts at the output   instead of 4 volts RMS and uh so we will consider  it good the transist I ended up using now I chose   between a bin of I have the exact replacement and  I have others and uh those are HP transistor 1854 0535 and supposed to be a little lower  uh frequency cut off but they worked   way better so and those are the one that  ended up there and gave me my Boost in   power so hopefully that will make things better  so the assembly is back in there and we'll see   if we have regain any power it's not super  good so nothing that Mi me jump up and down   uh let's let's have it warm up it might not be  exactly at 5 MHz yeah probably is not because   it was cold so it's probably off the tune  and it might increase a little longer than   a few minutes later yes and as it warmed up  it has indeed gone higher so now it's at 20   which is supposed to be the minimum normal it's  a lot better than 0.2 0.3 that we had before so   let's see if we can lock it so where are we  do we have beam I you have a little bit beam ey yeah it's locked [Music] so and it is still cold so that  I don't have that much beam current but I have   more than before my multiplier is now within  range and so I think that's the maximum we can   do on this one and we'll see if it allows us to  run a longer term stability experiment moments   later and look at that our uh beam current  is also much higher so we're getting a lot   more signal out of the tube this is still at  20 that's the multiplier output that is good   I think we uh restored it to where we had it  before so I think it's time for round two of   experiments with the clocks and see if we can  find the long-term stability between two clocks n

2025-01-24 05:06

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